1. Field of the Invention:
The present invention relates to a method of preparing pyromellitic acid dianhydride.
2. Description of the Prior Art:
The manufacture of pyromellitic acid dianhydride (PDMA) is conducted on a large scale by two different methods. One method is a liquid phase oxidation in which 1,2,4,5 tetraalkylbenzene or 2,4,5 trialkylbenzaldehyde reacts with nitric acid or a gas containing oxygen in the presence of a halogen-containing metal catalyst. This reaction does not yield the desired dianhydride, but the free acid, which must be dehydrated in a subsequent step to form the dianhydride. The dehydration step and substantial problems of corrosion make this method expensive and not very attractive.
The second method is a gas phase oxidation in which 1,2,4,5 tetraalkylbenzene, together with air in the gas phase is passed over a heterogeneous catalyst and the dianhydride desublimates when the reaction gas is cooled. In an improvement of the process as described in DE-PS No. 23 62 659, the reaction gas, after initially cooled to a temperature still above the condensation temperature of the PMDA, is further reduced in temperature over cooled surfaces controlled by thermostats to desublimate the PMDA. The temperature of the cooling surfaces must be below the condensation temperature of PMDA, but above the condensation temperature of byproducts of the reaction.
The disadvantage of this and similar methods, which use cooled surfaces to desublimate the PMDA, is that large cooling surfaces are necessary.
Another way in which PMDA can be desublimated from the reaction gas is to additionally cool the reaction gas by direct contact of said gas with a cold gas. Thus, for example, as shown in French Pat. No. A 2 082 822, cold air is added to the reaction gas in two successively arranged tubes. Except for an improvement of the product purity from 95% (without contact of the reaction gas with cold air) to 97% (with contact of the reaction gas with cold air where the ratio of reaction gas to air is 1:1), this process provides no improvement in the conditions of the process. In this disclosed method the wall area of the tubes is additionally a site for desublimation since it serves to conduct heat away from the product. Moreover, the surface of the wall, because it is not actively cooled is much too large for an industrial installation. Thus, for example, in a system having a throughput of 1 t of durene/hr., the wall area amounts to approximately 125,000 square meters. Upon introduction of cold air into the reactor, the method of French Pat. No. A 2,082,822 improves the separation of PMDA from the reaction gases upon its deposition on the reactor wall. This system also has the disadvantage, however, that the apparatus more rapidly clogs during the reaction than normally is the case. Because, as a rule, this method requires a significantly larger, usually doubled quantity of waste gas to be cleansed in comparison to the previously described method, the problem of eliminating the byproducts from the waste gas is compounded. In addition to these disadvantages is the fact that for the total process a double quantity of air, one part for the reaction, and one part for the desublimation, is used, which makes the method more expensive. For all of these reasons, this method has never been utilized in industry.
The problem of fractional desublimation of a material from a reaction gas is not only a problem with PMDA, but also is a problem which is encountered in the manufacture of some other materials. Thus, for example, British Pat. No. 1,081,579 describes a method and an apparatus for the fractional desublimation of terephthalic acid. In this method the reaction product is obtained through the direct cooling of a gaseous material under desublimation conditions, wherein the temperature of the gaseous material must reside below the condensation temperature of the product to be separated, but above the condensation temperature of the byproducts produced in the reaction. In the process the wall surface of the desublimator is adjusted to a temperature above the selected condensation temperature. In order to cool the reaction gas, water in its liquid form is injected into the gas as the preferred cooling agent. This cooling technique however, is not possible in the desublimation of PMDA, because in addition to the PMDA, byproducts would also condense on the water droplets before they completely evaporated. This problem is further compounded by the problem that with the large quantities of water or steam required at the temperatures of PMDA desublimation, the PMDA would be hydrolyzed to the acid. If the water were replaced with a cooling gas, then, when one calculates the gas needed to achieve the same cooling effect of quantities of water, very large quantities of cooling gas would be required, because a portion of the cooling gas would be heated by the walls as a result of the long period of time, 40 sec., that it remains in the condensation zone and therefore would no longer be capable of cooling the reaction gas. These long delay periods of the gas mixture in the condensation zone, in an attempt to transfer this method to the manufacture of PMDA, would also result in apparatus dimensions that would not be managable in industry, and the very large quantities of waste gas would render any sensible cleansing of the waste gas impossible. This method therefore cannot be employed in the manufacture of PMDA.
A need therefore continues to exist for an improved method of isolating pyromellitic acid dianhydride from the gases discharged from the oxidation of tetraalkylbenzene compounds.